1. Field of the Invention
The present invention relates to an ultrasonic vibrator device for ultrasonically cleaning a workpiece, and more particularly to an ultrasonic vibrator device having ultrasonic vibrators which are to be placed in a deaerated cleaning solution sealed in a cleaning tank for applying ultrasonic energy to ultrasonically cleaning a workpiece put in the deaerated cleaning solution while the deaerated cleaning solution is being pressurized.
2. Description of the Prior Art
Heretofore, there have been known ultrasonic vibrators which are to be placed in a cleaning solution supplied to an ultrasonic cleaning tank. More specifically, the ultrasonic vibrators are held in a sealed container in the ultrasonic cleaning tank, and a voltage is applied to the ultrasonic vibrators to vibrate the ultrasonic vibrators for thereby radiating ultrasonic energy into the cleaning solution to clean a workpiece immersed in the cleaning solution. The sealed container is made of an SUS plate having a thickness of about 3 mm such that the ultrasonic energy can efficiently be radiated therethrough into the cleaning solution when the ultrasonic vibrators are vibrated. The surface of the sealed container where the ultrasonic vibrators are mounted is plated with a hard chromium layer having a thickness of 20 .mu.m or greater in order to prevent the sealed container from being eroded by the cleaning solution.
When the ultrasonic energy is radiated from the ultrasonic vibrator into the cleaning solution, the cleaning solution is cavitated, and the workpiece is exposed to shock waves or microjets that are produced when the cavitation is collapsed. Foreign matter or burrs can be removed from the workpiece by those shock waves or microjets. For efficiently cleaning workpieces, it is necessary to provide conditions which facilitate the generation of the cavitation in the cleaning solution.
It is known that the cleaning solution can be cavitated more easily if the concentration of a gas dissolved in the cleaning solution is lower. If the concentration of a gas dissolved in the cleaning solution is too high, then the dissolved gas is converted into bubbles by the ultrasonic energy radiated by the ultrasonic vibrators, and the cleaning solution is less cavitated as the ultrasonic energy is absorbed by the bubbles. Therefore, when such bubbles are produced in the cleaning solution by the dissolved gas, the cleaning of the workpiece is essentially carried out only by the bubbles, but not by the ultrasonic energy.
The inventor has found out that the cleaning solution can more easily be cavitated when the cleaning solution is subjected to a suitable static pressure, and proposed an ultrasonic cleaning apparatus which applies a static pressure to a deaerated cleaning solution while a workpiece immersed in the deaerated cleaning solution is being ultrasonically cleaned (see Japanese patent publication No. 4-46637). The proposed ultrasonic cleaning apparatus has a sealing means for sealing a cleaning solution in a cleaning tank from the atmosphere, and a pressurizing means for applying a static pressure to the cleaning solution in the cleaning tank. Since the cleaning solution in the cleaning tank is sealed from the atmosphere, air finds difficulty in getting dissolved in the cleaning solution. The cleaning solution sealed in the cleaning tank can easily be held under a suitable static pressure by the pressurizing means. Therefore, the appropriate conditions for generating the cavitation in the cleaning solution are provided by the proposed ultrasonic cleaning apparatus.
However, though strong shock waves are produced upon collapse of the cavitation as the cavitation is easily developed, the wall surface of a sealed container housing the ultrasonic vibrator tends be strained by the static pressure applied thereto, causing the ultrasonic vibrators to be peeled off the surface of the sealed container. When the ultrasonic vibrators are peeled off, they fail to oscillate properly. The strained sealed container is apt to develop cracks in welded regions thereof, allowing the cleaning solution to enter the sealed container and causing a dielectric breakdown of the ultrasonic vibrators. The sealed container may be prevented from being strained by increasing the thickness of the wall thereof. However, the increased wall thickness hampers the transmission of vibrations from the ultrasonic vibrators, resulting in a reduction in the ultrasonic radiation efficiency.